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Salt Bridges in Voltaic Cells

  1. Nov 3, 2007 #1
    Iam doing an experiment on voltaic cells in particular Iam studying the effect that the thickness of the salt bridge has on the emf of the cell over time.

    iam thinking that the thicker the salt bridge the longer it will last. I think that will happen because maybe there is more surface area for electrons to move through it. But i totally made that up...Iam not sure if that is scientifically correct.

    So any suggestions anybody??
  2. jcsd
  3. Nov 5, 2007 #2
    I'm sorry I've been searching on the internet for almost 2 hours and could not find anything
    based on the thickness of the salt bridge at least non of my level of understanding. Most of the stuff I've found is based on the experiment conducted by universities on salt bridges


    You can try that URL and look under the the link that says [Electrochemical reference electrode - Patent 4002547] read the description below, and if that has anything to do with what you are looking for then go a head and look into it. Maybe you can understand it cause I seriously can't.

    As far as I know about the function salt bridges are:

    1) to allow the flow of ions so that the circuit is completed

    2) to prevent the two aqueous solutions from mixing. This will prevent displacement reaction between a more electropositive metal and the salt solution of the less electropositive metal from taking place.

    Sorry, I guess someone else will have to help you
  4. Nov 7, 2007 #3

    thanks so much for your help, I shall look at the site. You are correct about the salt bridges they basically keep the whole reaction neutral as the salt solution within the salt bridge like kinda cancels out any postive or negatives charges in either halve.
    I was thinking because when it does this whole neutralising thing the cell will just keep on running, soo...the thicker it is the longer it would run cause there is more room for ions to move through..haha but Iam quite sure thats wrong...
  5. Nov 7, 2007 #4


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    What do you mean thicker? Are you talking about one of those U-shaped hoses packed with liquid/gel and cotton balls? Or are you talking about a membrane?

    When you buy batteries, you'll notice they are rated in amp-hours (Ah) or miliamp-hours (mAh). The amount of charge is fairly constant, but the life of the battery changes when you change the flow rate of ions, which then changes the flow rate of electrons.

    Pretty much everything flows the same way. Gas, liquid, electrons, ions, whatever. They're all the same. If you make the salt bridge capable of having higher flow (like a wider U-shaped hose, or a thinner membrane), it would increase the flow rate of ions across it, which would increase the (maximum) amperage of the cell, which would decrease the life of the cell. The faster you burn gas, the quicker you run out of gas; simple concept. On the other hand, having a higher maximum flow means the resistance is less, which means the efficiency should go up. Should go up, but don't quote me on that.

    If you make the salt bridge longer (longer hose of the same width, or a thicker membrane), it lowers the efficiency of flow. As a result of lower efficiency, the maximum amperage drops, but the life of cell also drops because the amount of wasted energy goes up. Making a longer salt bridge is like stringing a bunch of extension cords together.

    Quick and dirty answer: thinner membranes and shorter hoses will give better cell life.

    edit: The thickness of the membrane shouldn't have any effect on the voltage itself. That is determined by concentration, temperature, and I think one other thing. The voltage may appear to drop significantly once you hit the maximum flow rate of the cell (if you try loading the cell with a light bulb or something). If you feel adventurous, you can try this effect on a AA battery (wear safety glasses while doing this). Run a short copper wire from + to - then connect a voltmeter to measure the voltage. The voltage will be much less than 1.5v. The battery might explode too, so be careful.
    Last edited: Nov 7, 2007
  6. Nov 8, 2007 #5
    By thickness I mean like how many tissues thick the salt bridge is.
    Iam a little confused with what your saying, so basically is it that thicker the salt bridge the less time it will take for the cell to die because the electrons are flowing through faster?? Is that right...
  7. Nov 11, 2007 #6
    Don't run a short across the battery, this is not safe. Cheapo batteries can burst.
  8. Nov 11, 2007 #7
    So, I'm assuming that you're using filter paper as your salt bridge instead of a glass tube. The things that effect the conductivity of a filter paper salt bridge are:

    The concentration of the electrolyte solution you dip it in (to increase conductivity, increase the concentration of the electrolyte solution below its saturation point),

    The texture of the filter paper (smoother texture = higher conductivitiy)

    Absorbency of the filter paper (the more absorbant the filter paper is, the higher the conductivity)

    The size/thickness of the salt bridge usually won't matter up until a point (for example, unless you're using something really small like a thin string).
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